I am taking images and would like to ensure the time captured in the FITS file is accurate.  I notice astroberry default install isn't set up to use NTP servers. Timedatectl reports NTP Service: Inactive.
Should I install NTP and configure to get time from specific NTP servers or is the default configuration sufficiently accurate? 

Hi David,

I understand that Astroberry uses "chrony", not ntpd, for the NTP protocol.  "sudo systemctl status chronyd" should show if this service is running.
chrony is running on my astroberry but timedatectl does indeed report "NTP Service: Inactive".  I do not know why this is.

Anyway, as the regular Raspberry Pi has no battery backed realtime clock, the date would default to 1970-01-01 on power up without an NTP client running and/or without network connection.
If you have a valid date/time and no battery realtime clock added, ntp must be running
Note: commercial RPi based astrocomputers will typically add a battery backed RTC  - valid date time is then no indication of running NTP. I have added a DS3231 based realtime clock to my  RPi (connects via I2C). for portable, off grid use.

As for accuracy: Without NTP the RPi clock is based on the processors crystal oscillator which is not temperature compensated. Time will drift with seconds per day. The DS3231 based RTC that I use, has a temperature compensated oscillator which (from memory) should drift less than a second per month. With NTP available, accuracy depends on many factors but will typically keep your clock within 10 of ms, but may drift up to 100 ms in congested, high latency networks. If time accuracy requirements are more strict (asteroid occultation measurements?) you could use GPS as a highly accurate local time source,

If you are going to use a GPS device , please note that USB3 signals can cause significant interference with the GPS signal (just as the widely reported interference that USB3 causes with 2,4GHz Wifi). I use a GPS dongle and place it as far as possible from RPi and USB3 cabels and camera. Even then I sometimes need to disconnect my USB3 camera to get initial GPS lock - but that is topic for another discussion.

Hope this helps,
Erik
 


 

Hi Erik. Yes your reply helps. Thanks for the tips. 'sudo systemctl status chronyd' indicates the clock is set on boot up and IP addresses of 3 servers are referenced. I am no expert but from I have read chronyd seems appropriate for an Astroberry client and is a later development to NTP although it uses the underlying NTP protocol. I normally turn off the raspberry pi after an observing session so it boots up regularly and must set the clock each time. I don't know how often chronyd checks when Pi is running? I guess there is a log that captures when the time is adjusted or a config setting that specifies how often it checks or maybe it works this out for itself based on how fast it drifts. Anyway I am assuming the system adjusts the time and doesn't drift with the crystal and I would have hoped the Astroberry developers would have flavoured raspberry with the best set up? It's just a case how accurate . To fractions of a second I hope.

The other factor possibly more significant is how accurate the fits file captures the start of the exposure because this is the time used in later analysis.

I have an observatory so don't change location but I notice Ekos does have GPS and interested in your suggestion of using a GPS dongle. Will it give me significantly more accurate time?